Photoimageable compositions useful as resists for forming printed circuits, printing plates, solder masks or the like have been used for some time now. The earliest photoresists were solvent-borne and solvent developable. The invention of aqueous developable resists represented an advance in reducing solvent emissions in the printed circuit board manufacturing plant and the environment in general. This can be seen from U.S. Pat. Nos. 4,100,047, 4,564,580 and 4,133,909, herein incorporated by reference, which describe aqueous coatings, also from U.S. Pat. Nos. 3,953,309 and 4,361,640, herein incorporated by reference, which teach aqueous developing compositions. The continued emphasis in reducing organic solvents has spurred the search for waterborne photoresists which are formulated and applied as aqueous liquids with out the addition of any organic solvents.
A critical requirement for photoresist, especially liquid photoresist, is the ability to evaporate the solvent carrier during a brief heat cycle to yield a tack-free surface. This permits intimate contact with the artwork used to define the image. Another requirement is the ability of the liquid photoresist to fully wet the surface and form a continuous film as the copper clad laminate used to manufacture printed circuit boards often has a variety of surface irregularities. These requirements have limited the development of successful waterborne products of this type.
U.S. Pat. No. 5,045,435, herein incorporated by reference, describes a waterborne photoresist composition which is developed in alkaline aqueous solution. The composition comprises a multi functional monomer, a photo initiator and a latex of water-insoluble carboxylated acrylic copolymer. To stabilize the composition and adjust the viscosity, this patent teaches neutralizing the latex polymer to at least 25% with a base, such as ammonia, another amine, or sodium hydroxide.
A liquid such as a waterborne photoresist liquid will only wet and spread over a surface if the forces of attraction between the liquid and the substrate are greater than the cohesive forces within the liquid. Accordingly, the surface energy of the liquid must be at or below the surface energy of the substrate. Thus, to coat a copper substrate having a surface energy of 35 dynes/cm, the surface energy of water, which has a surface energy of 72 dynes/cm, must be reduced. U.S. Pat. No. 5,045,435 teaches that this can be accomplished with the aid of a suitable surfactant.
A surfactant is a material with both hydrophillic and hydrophobic properties. Surfactants can be divided into three major chemical classes; hydrocarbons, silicones and fluorochemicals, these designations referring to the hydrophobic portion of the molecule. The hydrophobe is attached to a hydrophilic group, which can be anionic, cationic or non-ionic in nature.
According to U.S. Pat. No. 5,389,495, herein incorporated by reference, neither hydrocarbon surfactants nor silicon surfactants are particularly suitable in waterborne photoimageable compositions. Although all types of surfactants are found to sufficiently lower surface energy, it is found that hydrocarbon and silicone surfactants encounter problems with respect to dewetting of the copper substrate. Dewetting produces voids which results in an unsuitable coating. Furthermore, if the photoimageable composition is intended as a plating resist, silicone from the silicone-based surfactant can leach into the plating solution, creating potential bath failure. Of the major classes of surfactants, hydrocarbon based surfactants are found to be the least effective in reducing the surface tension of the liquid and do not produce defect-free coatings. Furthermore, hydrocarbon based surfactants contribute to the overall volatile organic components (VOCs) in the coating, which is contrary to the purpose of producing waterborne coatings.
U.S. Pat. No. 5,045,435 gives examples wherein fluorocarbon-based surfactants are used in waterborne photoimageable compositions but applicant has found that these compounds do not perform well and produce undesirable side effects. U.S. Pat. No. 5,389,455 gives examples wherein fluoroaliphatic oxyethylene adduct surfactants are used in waterborne photoimageable compositions. Applicant has found that these compounds are difficult to work with in a production environment and results can vary substantially with a change in surfactant concentration as small as 0.1% by weight of the surfactant.